This invention concerns a device for supplying weft thread in airjet weaving machines.
It is known that in airjet weaving machines the weft threads are inserted in the shed by means of a number of nozzles, such as a main nozzle, an auxiliary main nozzle and relay nozzles.
It is also known that in such airjet weaving machines the supply of air is adjusted as a function of measurements carried out on the weft threads, in order to let the insertion of each weft thread happen in optimum circumstances. To this end the nozzles mentioned above are connected to a compressed air source via one or more cutoff valves, whereby the cutoff valves are opened and/or shut off sooner or later in the weaving cycle as a function of the values measured and parameters entered. In order to influence the insertion behaviour of the weft thread very precisely, foremost to modify the moment of arrival of the weft thread, it is also known to throttle the supplied air by means of a controlled throttling element. An example of such a device is described in the Belgian patent No. 904.260. The insertion of the weft threads is done by means of a main nozzle moving along with the sley and an auxiliary main nozzle mounted fixed in front of the latter, whereby the blow air in the auxiliary main nozzle is throttled. To this end, the auxiliary main nozzle has a cone and a counter-cone which both fit into each other and between which the blow air can be conducted. One of the cones can be moved, or respectively removed from the other, such that the passage of the blow air can be modified.
Practical experience has shown that the use of an auxiliary main nozzle according to BE 904.260 for the present state of the art, with the ever-increasing machine speeds and the ever-extending range of yarn, leaves too little variation to modify the moment of arrival of the weft threads in relation to the weaving cycle. Current possible variations appear to take 8 to 14 milliseconds in a practical embodiment.
As throttling of the blow air according to the Belgian patent No. 904.260 is done with a throttling element which is inseparable from the auxiliary main nozzle, the blow air is throttled exclusively in the fixed auxiliary main nozzle, as a result of which a higher flow is led to the movable main nozzle during the throttling, resulting in greater traction here, which has a partly counter-productive effect.
The fact that the throttling is done on only one of the two main nozzles has the effect that, after the cutoff valve is shut off, all compressed air still present in the pipe between the cutoff valve and the main nozzles must escape through a smaller total opening, as a result of which the blow after effect after the moment that the cutoff valve is shut off, is extended.
It is also known that between the insertion periods of the weft threads a permanent airstream is provided in the main nozzles at low pressure, in order to hold the thread end present in said main nozzles. When the blow air is throttled in one of the main nozzles, this also causes the already low amounts of compressed air for holding the weft thread in the nozzles to be throttled even more.
Also, devices are known whereby the pressure of the blow air is controlled in the air receiver or buffer tank of the weaving machine. However, this technique has the disadvantage that each set pressure modification only shows after several insertions. It is therefore impossible to make adjustments during the insertion of a weft thread in order to modify the behaviour of this weft thread.